The objective of this research program is to develop and validate methods for studying in vivo somatic cell mutation in humans. Such assays are necessary for genetic risk estimates, as well as answering basic questions concerning the molecular nature of mutation in humans. Knowledge of the mechanism of mutation has important implications for understanding human disease. These studies aim to fully characterize "spontaneous" gene mutation occurring in vivo in human lymphocytes, and compare "spontaneous" with model alkylating agent induced gene mutation occurring in vivo in human lymphocytes. These objectives will be attained by achieving the following specific aim: (1) Completing population studies of in vivo hypoxanthine-guanine phosphoribosyltransferase gene (hprt) mutation in peripheral blood T-lymphocytes in normal humans, as well as defining in vivo amplifications of T-cell clones in normal individuals, ascertained as "outliers" with respect to mutant frequency values (> 50 x 10-6) and clonality of hprt mutants. (2) Characterizing wild type and hprt mutant T-cell clones recovered from the peripheral blood of normal individuals of surface marker phenotypes, specific T-cell receptor (TRC) gene rearrangement patterns, and hprt gene alterations. (3) Quantifying and characterizing L-phenylalanine mustard induced hprt mutation in vivo in peripheral blood lymphocytes of patients with multiple myeloma receiving such treatment. (4) Quantifying and characterizing in vivo hprt mutation in lymphocytes obtained from draining lymph nodes from breast cancer or myeloma patients and comparing findings with those obtained from peripheral blood in order to compare mutation in dividing versus non-dividing cells. (5) Developing a clonal assay for quantifying and characterizing gene mutations arising in vivo at the autosomal HLA locus in human lymphocytes, for comparison and with the X-linked hprt locus mutation frequency results. (6) Developing a clonal assay for quantifying and characterizing gene mutations arising in vivo in human B-lymphocytes, including definition of clonality as reflected by immunoglobulin (Ig) gene rearrangement patterns, for comparison with the T-lymphocyte hprt locus results.